Intermediate System-to-Intermediate System (IS-IS) is a routing protocol developed by the International Standards Organization (ISO). IS-IS is a link-state protocol and behaves much like the Open Shortest Path First (OSPF) protocol. IS-IS was developed as part of the Open System Interconnection (OSI) stack of protocols and uses OSI protocols to deliver packets and establish adjacencies. IS-IS routers need to be assigned OSI addresses, which they use as router identifiers to create network structure. IS-IS has been adapted to carry IP network information, and this form is called Integrated IS-IS. Integrated IS-IS has the most important characteristic necessary in a modern routing protocol: It supports VLSM and converges rapidly. It is also scalable to support very large networks.
IS-IS floods a network with link-state information to build a complete, consistent picture of a network topology. To simplify router design and operation, IS-IS distinguishes between Level 1 and Level 2 ISs. Level 1 ISs communicate with other Level 1 ISs in the same area. Level 2 ISs route between Level 1 areas and form an intradomain routing backbone. Hierarchical routing simplifies backbone design because Level 1 ISs need to know only how to get to the nearest Level 2 IS. The backbone routing protocol can also change without impacting the intra-area routing protocol.
IS-IS uses a single required default metric with a maximum path value of 1024. The metric is arbitrary and typically is assigned by a network administrator. Any single link can have a maximum value of 64, and path links are calculated by summing link values. Maximum metric values were set at these levels to provide the granularity to support various link types while at the same time ensuring that the shortest-path algorithm used for route computation is reasonably efficient. IS-IS also defines three optional metrics (costs): delay, expense, and error. The delay cost metric reflects the amount of delay on the link. The expense cost metric reflects the communications cost associated with using the link. The error cost metric reflects the error rate of the link. IS-IS maintains a mapping of these four metrics to the quality of service (QoS) option in a Connectionless Network Protocol (CLNP) packet header. IS-IS uses these mappings to compute routes through a public network.
IS-IS uses three basic packet formats: IS-IS hello packets, link state packets (LSPs), and sequence number packets (SNPs). Each of the three IS-IS packet types has a complex format that includes an 8-byte fixed header, a packet type-specific portion having a fixed length, and a packet type-specific portion having a variable length.
IS-IS uses type length value (TLV) parameters to carry information in LSPs. These TLVs make IS-IS extendable. IS-IS can therefore carry different kinds of information in LSPs. In an embodiment, IS-IS supports only CLNP. However, IS-IS was extended for IP routing with the registration of TLV 128 that contains a set of 12-octet fields to carry IP information.
In the IS-IS protocol data unit (PDU), there is a fixed part and a variable part of the header. The fixed part of the header contains fields that are always present, and the variable part of the header contains the TLV that permits the flexible encoding of parameters within link state records. These fields are identified by one octet of type (T), one octet of length (L) and “L” octets of value (V). The type field indicates the type of items in the value field. The length field indicates the length of the value field. The value field is the data portion of the packet. Not all router implementations support all TLVs, but they are required to ignore and retransmit the ignored types.
TLV 128 extends IS-IS to carry IP, in addition to connectionless network service (CLNS), routing information in the same packet. Several routing protocols use TLVs to carry a variety of attributes. Cisco Discovery Protocol (CDP), Label Discovery Protocol (LDP), and Border Gateway Protocol (BGP) are examples of protocols that use TLVs. BGP uses TLVs to carry attributes such as Network Layer Reachability Information (NLRI), Multiple Exit Discriminator (MED), and local preference.
There are network deployments where a link is unidirectional (UD) but there is a requirement for routing protocols to forward traffic over it. There is a need to extend routing protocols such as OSPF and IS-IS to run directly over unidirectional links (UDLs).
IP encapsulation is used for tunneling across a communication network of routers. An inner IP header is encapsulated by an outer IP header for the tunneling configuration. The outer IP header is added before the original IP header. Between them are any other headers for the path, such as security headers specific to the tunnel configuration. The outer IP header source and destination identify the endpoints of the tunnel. The inner IP header source and destination identify the original sender and recipient of the datagram. Each header chains to the next using IP Protocol values.
A send-only interface is a router interface that can only transmit. Likewise, a receive-only interface is a router interface that can only receive. The path from receive-only to send-only may be multi hop and can go further through other unidirectional links (UDLs). There is a need to support IS-IS over multi-hop return path without any IP encapsulation and in presence of other UDLs to eliminate the overhead and complexity of IP encapsulation.